The present invention relates to a device and method for bending a glass sheet.
Bent glass sheets can have various applications. They can be used in particular as glazing units in vehicles, e.g. automobiles, as windscreens, side windows, rear windows or roofs. These glazing units can be toughened or laminated. For such applications it is necessary that the bending device allows the glass sheets to be shaped in a precise manner without causing any optical flaws.
A technique is known in particular, wherein a glass sheet is transported on a roller conveyor through a tunnel oven, in which its temperature is brought to its softening temperature, then is passed to a bending station comprising a pressing operation, therefore also sometimes referred to as a pressing station. In the bending station the glass sheet is lifted from the conveyor by means of a frame with a shape corresponding to that which the periphery of the glass sheet is to be given.
The frame can either be discontinuous to be able to pass through the bed of rollers on which the glass sheet initially rests, or is preferably continuous, in which case the conveyor can be made up of roller wheels at this location. The frame then lifts the glass sheet in order to place it against the surface of an upper mould (also referred to as counter-mould or male press). The mould is complementary to the frame and therefore corresponds to the desired shape of the glass sheet.
Other variants comprise the same type of pressing operation. For example, the glass sheet can be placed on a frame before entering the tunnel and advance in this while being supported by the frame. The pressing on the upper mould is conducted by the same frame or a frame replacing this for lifting and pressing the sheet.
In some variants, the glass sheet can be applied against the upper mould by suction through this mould. This is achieved, for example, by means of a plurality of ducts connected to means that ensure a certain low pressure on the surface of the mould. These ducts connect, for example, with a central chamber under reduced pressure. Such an arrangement allows the glass sheet to be subjected to suction and therefore cooperates with the movement of the frame in shaping the glass sheet.
After pressing, the frame moves down again to the conveyor level. Following these techniques, either the frame retracts below the bed of rollers and thus places the glass sheet back onto said bed of rollers, or the frame resumes its path carrying the sheet.
In a variant, the lower frame moves down again while the glass sheet is still held against the upper press by the suction force thereof, a second “shuttle” frame is positioned below the glass sheet and suction is stopped so that the glass sheet is positioned on the shuttle. The periphery of this shuttle frame corresponds to the profile of the final bent sheet.
In all cases, the bent glass sheet is directed to the cooling or toughening station.
The type of bending technique described above is characterised by the fact that the pressing operation is conducted outside the oven or at least outside a temperature-controlled zone. This type of technique must therefore be considered as a cold method, this definition indicating the location of the pressing station outside a temperature-regulated chamber. It is also known to position a heating system in the upper press in order to compensate the temperature loss resulting from the bending being performed outside the oven. This heating system can be heating resistors, for example.
In another bending technique, the pressing operation is conducted in the oven. The presses can be heated by the oven in this case.
For each design of bent glass two complementary pressing elements have to be made: the solid upper mould, also referred to as the male press, and the lower frame, also referred to as ring or female press.
The male press is configured from a block that must be highly precise in shape. Moreover, in current practice, this press includes all the functional elements such as the means for ensuring suction of the sheet or the elements for heating the press when it is necessary to reheat the press to prevent the glass sheet from being subjected to a thermal shock. Therefore, in practice, the machining of this part is relatively complex, long and costly.
When the design of bent glass being produced on the bending assembly has to be changed for another design, the male press and female press must also be changed. Production is stopped to allow time to position the pressing element correctly opposite one another and time to reach the proper temperature of the presses. This time can be relatively long, particularly in the case of the male press, which is characterised by a certain thermal inertia mainly due to its mass.